The 2-alkyl-2-cycloalken-1-ones are useful substances as an intermediate for synthesis of physiologically active substances or perfume materials. As the method of producing the 2-alkyl-2-cycloalken-1-ones, there are conventionally known the method in which 2-(alkylidene)cycloalkanones are subjected to isomerization reaction by contacting with a solid acid catalyst in a vapor phase under heating condition (Patent Document 1), the method in which 2-(alkylidene)cycloalkanones are subjected to isomerization reaction using a platinum group metal catalyst under heating condition (Patent Document 2), the method in which 2-(alkylidene)cycloalkanones are subjected to isomerization reaction at a temperature of from 20 to 150° C. in the presence of a hydrogen halide or sulfonic acid (Patent Document 3), and the method in which 2-(alkylidene)cycloalkanones are subjected to isomerization reaction by contacting with a hydrogen halide in an alcohol solvent at a temperature of from 150 to 190° C. (Patent Document 4).
However, the method described in Patent Document 1 has such a problem that the vapor phase reaction requires complicated procedures. In the method described in Patent Document 2, there tends to occur such a problem that a double bond is reduced by a hydrogen gas used for activation of the catalyst to produce 2-alkylcycloalkanones as by-products, which results in poor yield of the aimed 2-alkyl-2-cycloalken-1-ones and therefore fails to obtain the 2-alkyl-2-cycloalken-1-ones with a high purity. Also, the methods described in Patent Documents 3 and 4 tend to suffer from the problems such as corrosion of reaction vessels used therein and need post-treatments owing to use of a strong acid.    Patent Document 1: JP-A 55-120533    Patent Document 2: JP-B 58-42175    Patent Document 3: JP-A 51-23240    Patent Document 4: JP-A 2000-327618